The team obtained data from the Danish Psychiatric Central Register and the Danish National Hospital Register for all individuals born in Denmark between 1 January 1955 and 30 November 2006 (over 3 million people in total). They found that 30,556 of these individuals developed schizophrenia or schizophrenia spectrum disorders, as diagnosed using the International Classification of Diseases, 10th Revision (ICD 10), between January 1995 and December 2012.

The crude incidence rate of schizophrenia among the study population was 3.59 per 10,000 person–years, and for schizophrenia spectrum disorders it was 6.80 per 10,000 person–years.

Patients who came into contact with hospital services for OCD, relative to those with no documented history of OCD, were at an increased relative risk of schizophrenia (incidence rate ratio [IRR]=6.90) and schizophrenia spectrum disorders (IRR=5.77).

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A diagnosis of OCD increased the risk of developing a schizophrenia spectrum disorder significantly more than a diagnosis of other childhood-onset disorders such as attention-deficit hyperactivity disorder (IRR=2.12) or bulimia nervosa (IRR=2.29).

The effect of an OCD diagnosis on risk of schizophrenia and schizophrenia spectrum disorders was relatively stable over time, with the degree of risk 1 year after OCD diagnosis similar to that after 12 years, at IRRs of 5.69 and 5.77, respectively. Thus, the authors say, “it is unlikely that the effect of OCD can be attributed to co-existing untreated psychosis alone.”

A parental diagnosis of OCD also increased the risk of schizophrenia (IRR=4.31) and schizophrenia spectrum disorders (IRR=3.10) in their offspring, and this effect was stronger than the risk associated with a parental diagnosis of any other psychiatric disorder, apart from schizophrenia or schizophrenia spectrum disorders themselves (IRR=4.97 and 3.15, respectively).

Writing in JAMA Psychiatry, Meier and colleagues suggest: “Future research is needed to disentangle which genetic and environmental risk factors are truly common to OCD and schizophrenia or schizophrenia spectrum disorders.”